A high-stakes British crime thriller where a protected witness vanishes and the detective hunting her is already being tracked.

In South Manchester, duty does not take days off.

Days before his wedding, DCI Rick Castle is pulled into the one case he cannot refuse—finding a missing protected witness who could finally bring down an untouchable Mancunian crime lord.

The trail leads him to the desolate Scottish Highlands and explodes into violence and conspiracy, revealing a suspected leak within the police. Cut off from support, and fearful he's being watched, Rick pushes on.

But the closer Rick gets to the witness, the closer his family gets to danger, turning his obsessive investigation into a fight he cannot walk away from—and a battle for survival he may not win.

If the case does not break him, they will go after the one thing that will.

Rule of Threes is book 5 in the DCI Castle series. Perfect for readers who love British police procedurals, relentless detectives, and gritty, high-stakes crime thrillers.

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Praise for the DCI Castle series:

★ ★ ★ ★ ★ Richly drawn characters… in a world of moral shades of grey.

★ ★ ★ ★ ★ I felt like I was there, on the streets, in every room.

★ ★ ★ ★ ★ A gripping crime novel… with complex characters and a story that keeps you hooked.

★ ★ ★ ★ ★ A fresh take on the crime genre.

★ ★ ★ ★ ★ Not many writers create a detective with vulnerabilities beyond the usual cop tropes.